Thymic stromal lymphopoietin (TSLP) is a cytokine that promotes the growth and development of B cells and their precursors in long term cultures. The focus of this study is an analysis of the signaling pathways activated upon engagement of the newly discovered TSLP receptor complex, which contains the unique receptor subunit TSLP-R, as well as the alpha chain subunit of the interleukin-7 (IL-7) receptor. This work will provide insight into the role of TSLP in B cell development and provide a comparison between the functions of TSLP and IL-7. This comparison will be especially intriguing because IL-7 has been implicated in B and T cell lymphomas and acute lymphoblastic leukemia. The specific aims of this proposal are three-fold. First, the identity of specific signal transduction molecules activated by TSLP receptor engagement will be determined with an emphasis on identifying the kinase phosphorylating Stat5 and the proteins that interact with the cytoplasmic domain of TSLP-R. These proteins will be identified using protein kinase inhibitors, co-immunoprecipitation studies, and the yeast two-hybrid system. Second, to understand how TSLP mediated signals are transduced, a detailed analysis of the sequence requirements of the cytoplasmic domains of the TSLP receptor complex will be performed. A strategy of generating mutant forms of the receptor chains and assessing the results of such mutations on TSLP mediated signals in cell lines and long term bone marrow cultures will be followed. Third, mice expressing a dominant-inhibitory mutant TSLP-R will be generated and analyzed to address in vivo the role of TSLP in B cell development.